1. Field of the Invention
The present invention relates in general to an inkjet print head, and more particularly to the construction of an inkjet print head having nozzles arranged in rows.
2. Discussion of Related Art
A prior art inkjet print head of on-demand type, as disclosed in JP-A-2002-137386 and U.S. Pat. No. 6,648,452, for example, includes a cavity unit 100 consisting of a plurality of plates 101-109 superposed on each other so as to define ink delivery passages, as shown in FIGS. 4A-4C showing a portion of the cavity unit 100 which portion is located on the right side of a widthwise center line of the cavity unit 100. These plates consist of: a nozzle plate 101 defining a plurality of nozzles 110 which are arranged in a total of four rows (only two of the four rows are shown in FIGS. 4A-4C); a base plate 109 defining a plurality of pressure chambers 111 which are also arranged in a total of four rows and each of which is held in communication at one of its opposite end portions with a corresponding one of the nozzles 110; two manifold plates 104, 105 defining common chambers 112 each of which is held in communication at one of its opposite end portions with an ink supply source; three spacer plates 106, 107, 108 interposed between the base plate 109 and the two manifold plates 104, 105 and cooperating with each other to define a plurality of ink delivery passages 113 each of which communicates with a corresponding one of the common chambers 112 and the other of the above-described opposite end portions of a corresponding one of the pressure chambers 111; a damper plate 103 defining damper chambers 114 which are located below the respective common chambers 112; and a spacer plate 102 interposed between the damper plate 103 and the nozzle plate 101. It is noted that each of the pressure chambers 111 is held in communication at the above-described one end portion with the corresponding nozzle 110 through a corresponding one of ink delivery passages 115 which are formed through the plates 102-108.
The inkjet print head further includes a piezoelectric actuator unit 116 which has piezoelectric ceramic plates, and internal electrodes in the form of common electrodes and arrays of individual electrodes formed on the piezoelectric ceramic plates such that the common electrodes and the individual electrode arrays are alternately superposed on each other. The piezoelectric actuator unit 116 and the cavity unit 100 are bonded together such that active portions existing between the common electrodes and the respective individual electrodes are aligned with the respective pressure chambers 111.
Each of the common chambers 112 formed in the manifold plates 104, 105 is elongated in a direction parallel with the rows of the nozzles 110 and the rows of the pressure chambers 111, and lies on a plane parallel with a plane defined by the rows of the pressure chambers 111 formed in the base plate 109. Each of the pressure chambers 111 is elongated in a direction perpendicular to the longitudinal direction of the common chambers 112. Each pressure chamber 111 has a portion which overlaps a corresponding one of the common chambers 112 as viewed in a plan view of the cavity unit 100, as shown in FIG. 4A.
Each common chamber 112 is held in communication at one of its longitudinally opposite end portions (not shown) with an ink supply source (not shown) via an ink supply hole (not shown) which is formed through the spacer plates 106-108. The common chamber 112 has, in its longitudinally intermediate portion, widthwise opposite ends 112a, 112b which are both parallel with the rows of the pressure chambers 111, so that a cross sectional area of the common chamber 112 is constant in the longitudinally intermediate portion. A major portion of each pressure chamber 111 overlaps the longitudinally intermediate portion of the common chamber 112. However, the cross sectional area of the common chamber 112 is gradually reduced in the other 112c of the longitudinally opposite end portions and its vicinity. That is, in the other end portion 112c and its vicinity, the cross sectional area is gradually reduced as viewed in a direction away from the above-described ink supply hole. This reduction of the cross sectional area in a region indicated by reference sign A is intended for facilitating discharge of bubbles (which tend to remain in the other end portion 112c of the common chamber 112) out of the common chamber 112 toward the corresponding pressure chamber 111 and nozzle 110.
Owing to the above-described arrangement in which the cross sectional area of the common chamber 112 is constant in a region indicated by reference sign B while the cross sectional area of the common chamber 112 is gradually reduced in the region A, each of the manifold plates 104, 105 is given a rigidity which is not constant. That is, the rigidity of each of the manifold plates 104, 105 is relatively high in the region A, while being relatively low in the region B.
In the cavity unit 100 as constructed as described above, when a pressure in the pressure chambers 111 is increased with activations of the active portions of the actuator unit 116 for ejecting an ink through the nozzles 110, the volume of each pressure chamber 111 located in the region A (in which the rigidity of each of the manifold plates 104, 105 is relatively high) and the volume of each pressure chamber 111 located in the region B (in which the rigidity of each of the manifold plates 104, 105 is relatively low) are changed differently from each other, so that the nozzles 110 exhibit respective ink ejecting performances which are different from each other and which are dependent on the locations of the respective pressure chambers 111.
In the inkjet print head disclosed in the above-identified Japanese and U.S. publications, a pair of open-space chambers 117 are formed in the manifold plates 104, 105, so as to be adjacent to a portion of the common chamber 112 which is located in the region A and in which the cross sectional area of the common chamber 112 is gradually reduced. The formations of the open-space chambers 117 are intended to reduce the rigidity of the manifold plates 104, 105 in the region A, namely, to substantially equalize the rigidity of the manifold plates 104, 105 in the region A and the rigidity of the manifold plates 104, 105 in the region B to each other.
The formations of the open-space chambers 117 cannot satisfactorily equalize the rigidity in the region A and the rigidity in the region B to each other. In the region B, since the pressure chambers 111 overlap the common chamber 112 equally with each other as viewed in the plan view of the cavity unit 100, the pressure chambers 111 can be deformed substantially equally with each other when the pressure in the pressure chambers 111 is increased as a result of activations of the active portions of the actuator unit 116. Further, in the region B, since each pressure chamber 111 except one of its longitudinally opposite end portions is located between the widthwise opposite ends 112a, 112b of the common chamber 112 as viewed in the plan view, each pressure chamber 111 can be deformed substantially evenly over its substantially entire length, upon activations of the respective active portions of the actuator unit 116. However, in the region A, since the pressure chambers 111 overlap the common chamber 112 differently from each other, as viewed in the plan view, the pressure chambers 111 deform differently from each other upon activations of the respective active portions of the actuator unit 116. Further, in the region A, since each pressure chamber 111 overlaps not only the common chamber 112 and the open-space chambers 117 but also thin partition walls 118 which are interposed between the common chamber 112 and the open-space chambers 117, the spacer plates 106-108 are partially supported by the thin partition walls 118, whereby the spacer plates 106-108 are partially restrained by the thin partition walls 118, from being downwardly deformed. Thus, each pressure chamber 111 in the region A cannot be deformed substantially evenly in its entirety, upon activations of the respective active portions of the actuator unit 116. Consequently, the ink ejection performance exhibited by each nozzle 110 located in the region A and that exhibited by each nozzle 110 located in the region B are different from each other, thereby leading to undesirable variation in the image formation performance of the inkjet print head.